Pebble Quicklime
Pebble Quicklime is derived from the heating of limestone to convert the calcium or magnesium carbonate into the respective oxide. Depending on the limestone source there are varying ratios of Calcium and Magnesium oxide. In addition to the Calcium/Magnesium oxide, there is an inert component that does not contribute to acid neutralization. Pure Pebble quicklime has a neutralization equivalent of 0.56 tons per ton of acidity compared to hydrated lime with a neutralization equivalent of 0.74 tons per ton of acidity. In addition, its higher density 55 to 60 pounds per cubic foot vs. 30 to 40 pounds per cubic foot allows for more alkalinity to be delivered on a per truck basis and it also allows for a smaller product silo.
Water powered mixers or “dosers” have been deployed to deliver pebble quicklime at sites where electricity is not readily available. These water-powered systems are sometimes referred to as semi-active. The solubility of pebble quicklime is somewhat limited, 1.33 g/L while hydrated lime is somewhat more soluble at 1.76 g/L. Before it can dissolve in water, pebble quicklime has to be converted from Calcium oxide to calcium hydroxide, a process called slaking.
The slaking equation is:CaO+H2O→Ca(OH)2  (1)It is an exothermic reaction commonly confined to a slaker where elevated temperatures promote the slaking process. Limited quantities of water are used to keep temperatures elevated. If excessive amounts of water are used, the lime is said to be “drowned” and the slaking reaction is inhibited with a coating of calcium hydroxide on the particle surface to restrict water penetration. It also restricts the reaction. This is the typical state of reaction at most semi-active treatment sites. To overcome this limitation, pebble quicklime is allowed to deposit in long channels where dissolution can proceed slowly. This can lead to low lime utilization at the treatment site due to un-dissolved lime buried in the channel or lime that has absorbed carbon dioxide from the air and converted back to calcite.Manor
The Manor mine is located in North Central Pennsylvania 12 miles northeast of the town of Clearfield. The mining took place in the Lower Kittanning seam in an up-dip direction. The mine discharge emanates from a wet seal installed about 2004. Under a consent agreement, Pennsylvania Department of Environmental Protection (PADEP) has been managing the operations a treatment plant at Manor since 2004. Prior to the recent modification, treatment consisted of calcium oxide addition using a water wheel (AquaFix) to regulate dosing. A mixing channel 300 feet in length with a vertical drop of about 20 feet was provided to allow for mixing and dissolution of the pebble quicklime. The treated water was allowed to cascade for aeration and then was allowed to settle in three baffled ponds before discharging. As a result of this process, significant quantities of lime were deposited in the mixing channel and in the first settling pond. This reduced the storage capacity of the settling pond and resulted in the disposal of large amounts of unused lime along with the cost of lime sludge removal.